This is a 371 of PCT/GB 94/00954 filed May 4, 1994.
This invention relates to novel conditioning processes useful in prolonging the effective life of functional fluids such as hydraulic fluids.
A variety of materials are utilised as non-aqueous hydraulic fluids and lubricants. The materials which are most commonly employed are triaryl phosphate esters, mineral oils and carboxylate esters. In use all of these materials degrade which degradation may manifest itself in the form of increased acidity of the fluid. As a result of this degradation the fluid must be replaced periodically. As this replacement is an expensive and time consuming operation a variety of reconditioning processes have been utilised in order to prolong the effective life of the fluid.
The various reconditioning processes which had been proposed for use with fluids comprising triaryl phosphates were reviewed by W D Phillips in Lubrication and Engineering Vol 39 (12), p766 to 780. A wide variety of solid materials including Fullers Earth, Attapulgus Clay, Activated Alumina, Diatomaceous Earth and Bentonite Clay have been used to filter the fluid. However, none of these processes proved to be ideal. There have also been proposals to utilise a macroporous anionic ion exchange resin as a filter preferably in combination with a cationic ion exchange resin. One disadvantage of using ion exchange resins to filter the fluid is that they are normally wet and their use introduces water into the fluid. This introduction of water is disadvantageous especially for fluids comprising esters such as triaryl phosphates, trialkyl phosphates, alkyl aryl phosphates and carboxylate esters as acids are produced by hydrolysis of the ester. There have been proposals to alleviate the increase in acidity resulting from hydrolysis of the fluid by incorporating a means for removing water from the fluid. The use of a vapour extractor acting on the main tank holding the fluid and a molecular sieve filter have been recommended although neither has proved ideal in use on a commercial scale.
The use of molecular sieves is less effective for ester based fluids as the acids produced by hydrolysis tend to remove metal ions from the sieve which in turn can increase the amount of particulate material deposited in the fluid and adversely effect the fluids surface active properties.
We have now discovered a novel reconditioning process for non aqueous functional fluids which utilises an anionic ion exchange resin in combination with a vacuum dehydration system. Surprisingly, such procedures are effective in prolonging the life of a fluid or in reconditioning a degraded fluid.
Accordingly, this invention provides a process for the treatment of a non-aqueous functional fluid which comprises contacting said fluid with an anionic ion exchange resin in the presence of sufficient water that the acidity of the fluid may be reduced and subsequently removing water from said fluid by vacuum dehydration.
The processes of this invention may be utilised not only to maintain the condition of a fluid but also to recondition a degraded fluid. The process may be operated on a batch or a continuous basis or any combination of these two approaches.
In a preferred embodiment a portion of the fluid used in a particular apparatus will be passed through a bypass, loop and the process will be applied to the fluid passing through the loop. The treated fluid will be combined with the main body of the fluid. The process may be operated continuously or intermittently in response to an analysis of the condition of the main body of the fluid.
The contact between the fluid and the anionic ion-exchange resin may conveniently be carried out by passing the fluid through a bed of the resin. Conveniently the resin can be packed into a cartridge which is inserted into a circuit through which the fluid can be pumped. In the preferred embodiment this circuit will be a bypass loop through which a minor portion of the fluid is passed.
Commercially available forms of the anionic ion exchange resins normally contain relatively large amounts of water. Some of this water is picked up by the fluid and removed in the dehydration step. These commercial resins may be and preferably are utilised in the processes of this invention. The water content of the resin may be reduced prior to use but this is not normally necessary. We have discovered that the resin bed remains functional in the processes of this invention for long periods without the need for its regeneration. During the operation of the process the resin may become ineffective with the result that the acidity of the fluid is not reduced to or maintained at the desired level. In this circumstance it may be necessary to employ a fresh resin bed.
The resin can be used either in the commercially available forms which are relatively wet or it can be predried so as to reduce its water content. The water content of a predried resin should not be reduced to a point at which it is rendered ineffective or degraded. In general we prefer to utilise resins which contain from 1 to 20 preferably from 3 to 15% w/w of water. The use of predried resins may be preferable since this minimises the initial increase in the water content of the fluid when it is contacted with the resin.
After contact with the resin the fluid will contain a quantity of water and this is reduced by a vacuum dehydration process. Such dehydration processes can readily be effected using commercially available equipment such as the HIVAC 124 Oil Purifier and the HSP 181 Series Oil Purifiers sold by Pall Industrial Hydraulics Limited. The dehydration should be carried out at a temperature which is sufficiently low to avoid degradation of the fluid. In the case of triaryl phosphate based fluids the dehydration will preferably be carried out at a temperature of less than 75xc2x0 C. The dehydration step need not remove all the water although it is preferred that as much water as is reasonably practical is removed.
The dehydration step is preferably applied to the fluid shortly or immediately after it has been contacted with the ion-exchange resin. In this preferred embodiment the degradation of the fluid is minimised. The dehydration equipment is advantageously and conveniently situated on the by-pass loop, immediately downstream of the resin bed. The dehydration step is more effective when applied to the relatively small volume of fluid using this arrangement.
The use of an anionic exchange resin treatment in combination with a vacuum dehydration step has been discovered to reduce the acidity of the fluid and may also serve to increase its volume resistivity. The treatment process also tends to increase the clarity of the fluid and to reduce the concentration of metal ions in the fluid. The treatment may also serve to improve the water separability of the fluid.
The useful life of the fluid is prolonged as a result. In a preferred embodiment the fluid is subjected to a vacuum with a pull of from 650 to 950 mbar (110 to xe2x88x92200 mbar absolute pressure) and more preferably to a pull within the range 750 to 880 mbar (10 to xe2x88x92120 mbar absolute pressure).
The processes of this invention may usefully be employed to treat any of the major classes of functional fluid in particular fluids based upon triaryl phosphates, trialkyl phosphates, alkyl aryl phosphates, mineral oils, carboxylate esters and water/glycol fluids. The processes of this invention find particular application in the treatment of triaryl phosphate based fluids.
The triaryl phosphate may be derived from natural or synthetic phenols. Examples of phosphates derived from natural phenols include mesityl phosphate, tricresyl phosphate, trixylenyl phosphate and cresyl diphenyl phosphate. The synthetic phenols are most preferably those prepared by the reaction of phenol with an alkylating agent having three or four carbon atoms. The product of such a reaction may comprise a mixture of phenols and alkylated phenols. Triaryl phosphates produced by the phosphorylation of such a mixture may also be used in this invention.
The carboxylate esters which may usefully be treated according to this invention include the trimellitates, diesters eg., adipates, sebacates, and esters of trimethylolpropane and pentaerythritol. The processes of this invention are preferably used to maintain or return the properties of the fluid to the levels preferred by the user. In particular the water content of the body of the fluid will preferably be maintained or reduced to less than 0.1% w/w and more preferably less that 0.05% w/w. The neutralisation number of the fluid will preferably be less than 0.2 mgKOH/gm and more preferably less than 0.1 mgKOH/gm.
The applicants have also discovered that the processes of this invention offer an additional advantage in that the concentration of metal ions in the fluid, most notably, sodium, calcium, magnesium and aluminium can be controlled, or in the preferred embodiments reduced. The processes of this invention are preferably the sole process used to treat the fluid. They can, however, be used in combination with other known processes or with particular elements of those known processes. In particular, the fluid may also be contacted with a cationic ion exchange resin or with a filter comprising both an anionic and a cationic ion exchange resin. However, in general the use of a cationic resin will detract from the advantages of the processes of this invention and is thereby less preferred.
Functional fluids based on those various classes of compound will normally be formulated with minor proportions of other ingredients such as antifoaming agents and oxidation inhibitors.
The suitability of any particular anionic ion exchange resin for use in the processes of this invention may be determined by routine experiment. Those which are ineffective in reducing the level of acidity in the fluid are not useful. Likewise any resins which are not stable in the presence of the fluid are preferably not used. Examples of ion exchange resins which are useful in the processes of this invention include those sold under the Trade Marks AMBERLYST A-21, AMBERLYST IRA-68 and DOWEX 400 LB UG. The physical form of resin may affect its performance in the processes of this invention. In particular we have discovered that uniformly sized resin particles (such as are sold as monospheres) may provide improved performance and their use forms a preferred aspect of this invention.